The prior art includes CVT assemblies that include three basic components of the clutch system—the primary or primary engine or drive pulley, the secondary or driven pulley and the belt. In operation the belt must slip compliantly and transmit full engine power with virtually no slippage from the primary pulley to the secondary pulley in respective operating modes. Unlike an automotive transmission system, the belt and pulley mechanism on a snowmobile or other CVT serves as both the clutch and the gear system. In making the transition from a starting gear ratio to the final “top gear,” the snowmobile uses two belt pulleys which have the ability to open and close axially (sideways) so that a specially designed belt can ride in various positions in each pulley. A representation of such an assembly is shown in FIG. 2.
The primary or drive pulley has a spring pressure that holds the pulley halves or sheaves apart when the engine rpm is low to facilitate de-clutching. As rpm increases, the centrifugal forces created by clutch weights overcome the spring pressure and close the pulley sufficiently to engage the belt and start transmitting power. The squeezing force created by the clutch weights continues to increase as the engine rpm increases. Assuming the sled is properly geared and has sufficient horsepower, the transition from low range to high range will continue until top speed is obtained. Various springs with different rates and lengths determine the pretension and can be used to change the engagement RPM and “refine” the shifting characteristics. The higher the spring pretension, the higher the engagement speed.
The clutch weights are the speed-sensing component of the primary pulley. As the engine rpm increases, the clutch weights swing out against the rollers in the spider tower, generating a force that quickly overcomes the spring pretension. As the engine speed increases, the clutch weights and their geometric relationship with the spider tower provide sufficient squeezing force to allow even the extreme horsepower of the “hyper sleds” to be transmitted with very minimal slip. It is estimated that the squeezing forces on the belt for the hyper sleds are in excess of 2000 lbs.
The design of the clutch weight shape and its geometric relationship with the spider is extremely complex. The shape or profile of the flyweight roller contact surface can modify the belt engagement speed and determines the unique shift curve required for each combination of engine, sled, and riding condition. Greater flyweights weight results in higher squeezing force and higher horsepower capability.
The secondary pulley also contains a spring, but unlike the primary pulley spring, it does not directly contribute to squeezing force. It functions in torsion (or twist) which assists the operation of the torque-sensitive cams. The twist creates some preload on the belt through the cam system and is quite important in controlling the back-shifting of the system when the throttle is released. Various springs are available for tuning purposes. The spring torque can be adjusted by placing the spring end in different adjustment holes in the pulley. In the secondary pulley, the torque sensitive cams are the heart of the feedback system. All of the transmitted power is fed through these cams.